Earth Fuses: Why Do They Blow?

This page deals specifically with the Pro-120, but other controllers have a fuse in the earth, or can blow a pot earth connection. This page explains how.

Wiring faults can cause the fuse in the earth track of the Pro-120 (or any other controller) to fuse. There is a deliberate weak point – but if the fault current is unfortunate, it can take out the long earth track shown in red below.

If the track blows, nothing else is usually damaged and it is quite safe to simply link out the track with a length of insulated wire, as shown by the green line. Of course, we can do this neatly, but standard service charge applies.

Customers do not usually admit to how they blow this track! However it can only be blown by high current (10 amps plus) in this track. The controller, properly wired, can never do this.

So, consider the diagram below.

The battery, and controller B- connection are both connected to chassis at point PE. No problem so far. But there is also a chassis connection at the control input – possibly a connector (such as a DIN socket) with a metal body is used and the body is a connection for the pot 0v line.

Normally a high current (peaks of 150 amps or more with the Pro-120) flows in the B- connection. But if the wire from B- to chassis earth is bad (possibly a bad crimp) it will have a high resistance. The controller will still try to draw current and that current will try to flow in the path of least resistance, so it will flow through the chassis (pecked arrowed line) to the connector earth (CE) and so through the pot earth where it will blow the track shown above if it is of just the right magnitude.

The magnitude of the fault current will depend on how bad the power connection is and how much current the motor is drawing, so the fault current will gradually rise as motor load increases. The earth fuse track is designed to blow at a high current overload, but a lower current will cause hating in the copper track and the red track above is long and not near any large thermal mass, so if the current is just wrong, it will heat, lift off the board and then fuse!

Another possibility with some controllers (not the Pro-120) is if the controller is connected up in the wrong order when an input earth short is present. Everything except the battery negative is connected correctly to the controller, but the battery negatine is still connected to chassis. If now the battery + is connected before the battery -ve, then there can be a problem: there are large capacitors in the controller which will now charge up through the input chassis earth connection. This may well blow the earth fuse track on, for instance, the VTX controllers.

The Pro-120 has a ‘trickle charge resistor’ which limits this charging current so it will not blow a fuse track.

So you should never under any circumstances use the chassis of an input connector as a live connection, or allow any other chassis connection to the controls of a motor controller!

Other pages relevant to this subject

• Machine wiring
• Pot min connection has fused (Uni controller)
• 4QD series controllers. Earth track fuse and Earth loops
• Pro-120 and VTX. Problems when double heading